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Full text of "The Cambridge Natural History"

82                               RANGE   OF   MARSUPIALS                            CHAK
A third cause of more or less limited range is the barrier
cine to competition. If the ground is already taken up, there is
no room for new immigrants. There is obviously a limit to the
number of Antelopes or Deer that can graze upon a given tract
of grassy plain. These two groups of Ungulates illustrate the
matter well: the Antelopes are African and Indian, especially
the former, while Africa has no Deer at all; America, on
the other hand, has plenty of Deer but no Antelopes, save the
Prong-horn. The more nearly akin the two species or groups
of species are, the fiercer will be the competition; for a near
kinship will at least often imply similar habits, the need for
similar food, and other likenesses which will prevent both from
successfully occupying the same tract of country. The remark-
able fauna of Australia is believed to afford an example of this.
In that- country the prevalent inhabitants are the Marsupials.
The Monotremes are found there also, and nowhere else save in
New G-uinea and Tasmania. The remaining mammals are in-
conspicuous; they embrace a few Rodents and Bats, and the
doubtfully indigenous Dingo-dog. Now the Marsupials are
fitted to every variety of life. We have the grazing Kangaroos
and Wallabies, the burrowing Wombats, the arboreal Phalangers,
and the carnivorous Dasyures. In the second place, it is an
unquestioned fact that the Marsupials are an older race than
are the existing Eutherian mammals; they were the dominant
mammals during the Secondary epoch. At that time they were
more widely distributed than at present. In most parts of the
world they are now absent, since they have been successfully
ousted by the more highly organised groups of Eutheria. But at
that period, when the higher Eutheria were in the ascendant,
Australia and the islands to the north became cut off from Asia,
and thus became freed from inroads of Eutheria, which were
partly prevented by the physical barrier of the sea from effect-
ing a settlement, and partly perhaps prevented owing to the
ground being already taken tip by the Marsupials. Likeness
of habit gave the older inhabitants victory in the struggle for
existence.
The general statements that have been here made are in
accord with current opinion upon the factors of geographical
distribution. But the past range of animals appears to be
less consonant with the received viewa In the Tertiary